Format String in Echo Go

Format string vulnerabilities in Echo Go applications occur when user-controlled input is passed directly to formatting functions like fmt.Sprintf, fmt.Printf, or log.Printf without proper sanitization. In Echo Go, these vulnerabilities often appear in error handlers, logging middleware, or response formatting where developers inadvertently expose internal state or allow attackers to read memory contents.

A common Echo Go pattern that introduces format string vulnerabilities is using fmt.Errorf with user input:

func getUserHandler(c echo.Context) error {
    id := c.Param("id")
    user, err := db.GetUser(id)
    if err != nil {
        // VULNERABLE: id is user-controlled and passed directly to fmt.Errorf
        return echo.NewHTTPError(http.StatusInternalServerError, fmt.Errorf("error fetching user %s", id))
    }
    return c.JSON(http.StatusOK, user)
}

In this Echo Go handler, an attacker could craft the id parameter to include format specifiers like %x or %p, causing the application to leak memory contents or crash. The vulnerability exists because fmt.Errorf processes the id string as a format string rather than treating it as literal data.

Echo Go middleware that logs request data can also introduce format string issues:

func loggingMiddleware(next echo.HandlerFunc) echo.HandlerFunc {
    return func(c echo.Context) error {
        // VULNERABLE: user agent is logged without sanitization
        log.Printf("Request from %s", c.Request().UserAgent())
        return next(c)
    }
}

While this appears safe, if the user agent contains format specifiers, it could cause log injection or information disclosure. Echo Go's default logging configuration doesn't sanitize these inputs, making this a realistic attack vector.

Detecting format string vulnerabilities in Echo Go applications requires both static analysis and runtime scanning. Static analysis tools like golangci-lint can flag suspicious patterns, but they often miss context-specific issues in Echo Go's handler chains.

middleBrick's scanner specifically targets Echo Go applications by analyzing the runtime behavior of API endpoints. When scanning an Echo Go application, middleBrick tests for format string vulnerabilities by:

• Injecting format specifiers (%n, %x, %p, %s) into all string parameters and observing application behavior
• Checking for memory disclosure through error messages and logs
• Testing response formatting functions that might process user input
• Analyzing middleware chains for unsanitized logging

Using middleBrick's CLI to scan an Echo Go application:

middlebrick scan https://your-echo-app.com/api/users/123

The scanner will test the /api/users/{id} endpoint with various format string payloads and report if any lead to information disclosure or application instability. For Echo Go applications specifically, middleBrick checks the common patterns where format strings appear in error handling and logging middleware.

Echo Go developers can also perform manual testing by crafting requests with format specifiers:

# Test for %n (writes to memory - likely to crash)
curl -X GET "https://your-echo-app.com/api/users/%n"

# Test for %x (hex dump)
curl -X GET "https://your-echo-app.com/api/users/%x%x%x%x"

# Test for %p (pointer addresses)
curl -X GET "https://your-echo-app.com/api/users/%p"

If the application crashes, returns unusual error messages, or exposes memory contents, a format string vulnerability exists.

Remediating format string vulnerabilities in Echo Go applications involves using the correct formatting functions and implementing proper input validation. The primary fix is to use fmt.Sprintf with explicit arguments rather than passing user input directly to formatting functions.

Secure Echo Go handler pattern:

func getUserHandler(c echo.Context) error {
    id := c.Param("id")
    user, err := db.GetUser(id)
    if err != nil {
        // SECURE: use explicit formatting with literal string
        return echo.NewHTTPError(http.StatusInternalServerError, 
            fmt.Errorf("error fetching user %s: %v", id, err))
    }
    return c.JSON(http.StatusOK, user)
}

This pattern ensures the id parameter is treated as data, not as a format string. The %s in the format string is a literal format specifier that will display the id value safely.

For logging in Echo Go middleware, use structured logging or explicit formatting:

func loggingMiddleware(next echo.HandlerFunc) echo.HandlerFunc {
    return func(c echo.Context) error {
        // SECURE: explicit formatting with literal string
        log.Printf("Request from %s: %s %s", 
            c.Request().UserAgent(), 
            c.Request().Method, 
            c.Request().URL.Path)
        return next(c)
    }
}

Echo Go also supports structured logging through third-party libraries like zap or logrus, which eliminate format string concerns entirely:

import "go.uber.org/zap"

func loggingMiddleware(next echo.HandlerFunc) echo.HandlerFunc {
    logger := zap.L()
    return func(c echo.Context) error {
        logger.Info("request", zap.String("user_agent", c.Request().UserAgent()))
        return next(c)
    }
}

For Echo Go applications using templates or response formatting, always validate and sanitize user input before passing it to formatting functions. Consider using a whitelist approach for expected input formats rather than trying to sanitize all possible format specifiers.